I agree with the others above - it shouldn't take long, but might feel like a long time sometimes, so don't try to time yourself or count seconds or anything.

My advice for DEILD is to just relax and imagine the previous dream or the scene you want to be in or even just a piece of a dream (only focus on that + 'I'm dreaming' + naturally fall asleep). What's important is to stay relaxed and not over-think it... Increased heart rate is probably a result of trying to hard to imagine the scene, or trying to hard to fall asleep FAST, or any other sort of stressing out. Overall it's the same with all WILDs - better to just fall asleep and loose awareness than to over-stress / over-do it and jolt up as a result.

Plus, like gab said, I also find it more effective to not fully wake up... It takes some practice and not always possible, but if it happened that you gained awareness while waking up - don't wait to fully wake up - just start the DEILD.

A note about "imagining a scene" - I don't know how it is for others, but for me the shift from what I imagine to being immersed in the dream is very sudden. One minute the "imagined scene" looks totally imagined and not detailed enough, and suddenly it becomes completely real... So don't get frustrated with your imagination - it can happen suddenly so just give it time to happen.